An electron beam lithography system includes a pair of electrodes for electrostatically deflecting an uninterrupted beam of electrons away from the optical axis towards a stopper and thus “switch” the electron beam “on” and “off”. This process is commonly referred to as “blanking”.
Normally, the beam is deflected so as to hit a stopper which may be a part of the electron beam column or a block of metal specially provided for the purpose. However, a drawback of this technique is that primary electrons striking the stopper generate secondary and backscatter electrons which charge-up the part of the system through which the electron beam passes.
One solution is to provide a stopper in the form of an aperture or opening with a pocket into which the electron beam is deflected. Although secondary electrons are still generated, they are absorbed by the walls of the pocket. The smaller the aperture, then the fewer secondary electrons escape. However, as the aperture is made smaller, the more difficult it becomes to deflect the beam into the aperture. Therefore, a compromise is usually made by making the aperture small enough to reduce escape of secondary electrons, while keeping it large enough to aid deflection of the electron beam into the aperture.
Another solution is to apply a positive voltage to the stopper. When positively biased, the field draws secondary electrons towards the stopper. Although this technique helps to reduce secondary electrons, it can be difficult to implement since it requires the blanking bias and the collector bias to be switched simultaneously.
Ion beam systems suffer similar problems.
The present invention seeks to provide improved apparatus for blanking a charged particle beam.